Lead, which has useful electrochemical properties such as conductivity and can be readily processed, is employed in a variety of industrial products, including solder, batteries, and glass materials. When these lead-containing products are discharged as municipal wastes and industrial wastes and incinerated in an incineration plant, in some cases, high-concentration lead is detected in the incineration ash and fly ash discharged from the plant. Although the discharged incineration ash and fly ash are subjected to landfill disposal, there are concerns about elution of lead by, for example, rainwater around the landfill sites. As has been known, lead itself is very poisonous, and even a trace amount of lead is strongly harmful to the human body, causing malfunction of the nervous system and lead-poisoning symptoms such as anemia, headache, inappetence, and lead colic. Therefore, standards for pollution control on landfill sites and the like have been established, and agents and methods for preventing elution of lead have been developed, along with lead assay methods for leachate. Furthermore, since lead is strongly harmful to many other living organisms, standards such as environmental quality standards and wastewater quality standards have been established with respect to lead. Meanwhile, there is demand for a simple lead assay method which can be applied to environmental analysis performed in a variety of fields.
Generally, lead concentration is determined through a method employing an atomic absorption photometer or an ICP emission spectrophotometer. Such a method requires feeding of acetylene gas or argon gas and provision of an exhaust duct, and involves cumbersome operation. Therefore, the method is not suited for routine on-site environmental analysis and routine analysis carried out on incineration sites.
Meanwhile, absorptiometry, which determines the concentration of a substance in a sample by use of a colorimetric reagent, can be performed by means of a small, inexpensive apparatus through simple operations. Therefore, absorptiometry is suited for simple assay. The colorimetric reagent employed in determination of lead concentration includes porphyrin derivatives such as water-soluble porphyrin and porphyrin nucleus-incorporated polymer. As has been known, by use of such a porphyrin derivative, a trace amount of lead contained in aqueous solution can be determined.
However, environmental analysis solution samples generally contain a variety of miscellaneous matters other than lead. These miscellaneous matters are possible interference substances in high-sensitivity and accurate lead assay. For example, calcium, which is always found in the environment (e.g., in sea water, river water, tap water, industrial wastewater, or ash leachate), is present in aqueous solution as a divalent ion, as is the case with lead. Thus, calcium may serve as an interference substance in determination of lead through absorptiometry based on chemical reaction between a colorimetric reagent and lead. Particularly, since ash leachate contains calcium at a concentration as high as several tens to several hundreds of mM, difficulty is encountered in determination of lead in ash leachate through absorptiometry employing water-soluble porphyrin as a colorimetric reagent. In order to determine, through absorptiometry, lead concentration of a sample solution containing calcium, a preliminary treatment (separation of calcium from lead, or masking of calcium) must be performed.
For separating calcium from lead, there may be employed a technique employing a chelate resin predominantly containing iminodiacetic acid, which has strong affinity to heavy metals, including lead. Thus, when a lead-containing sample solution is caused to pass through a column or filter filled with or formed of such a chelate resin, lead can be selectively captured. Then, the thus-captured lead is eluted through passage of an acidic eluent, and the lead concentration of the eluate is determined. This technique requires passage of an eluent in an accurate volume many times by means of a pump or syringe, making the technique cumbersome. Therefore, the technique is not suited for a simple assay technique.
Ethylenediaminetetraacetic acid (EDTA) and ethylene glycol bis(2-aminoethyl ether)tetraacetic acid (EGTA) are known calcium-masking agents. However, these masking agents also capture lead. In order to mask high-concentration calcium, a masking agent must be added at high concentration. In this case, lead that is present at a concentration considerably lower than that of calcium is thoroughly masked. Therefore, neither EDTA nor EGTA can be employed as a calcium-masking agent in determination of lead through absorptiometry employing water-soluble porphyrin.
For the absorptiometric determination of lead in a sample solution containing calcium as an interference component, there has been reported a lead concentration determination method in which such a solution is treated with an agent containing calcium added in advance thereto and a porphyrin nucleus-incorporated polymer (Patent Document 1). When the method is employed, interference of calcium can be eliminated. However, since preparation of the reagent involves a polymerization step, considerable time and cost are required. In addition, the porphyrin skeleton which has been incorporated into the polymer exhibits a broadened peak, reducing the determination sensitivity. Therefore, in an assay of microamount lead at the environmental quality standard level or wastewater quality standard level, there is further demand for enhancement of the sensitivity of the assay.    Patent Document 1: WO 2006/011549